1. Field of the Invention
This invention relates to the field of semiconductor switching devices, and particularly to switching devices used in high-power switching circuits.
2. Description of the Related Art
Semiconductor devices are increasingly required to accommodate high currents and/or high voltages without failing. Many applications, such as pulse-width modulated motor-control circuits, require high-power switching devices. A number of devices have been developed to provide the high current and reverse blocking characteristics needed in a high power switch. The available devices offer various levels of performance for the primary parameters of interest, such as forward voltage drop VFD and safe-operating-area (SOA). SOA is defined as the current-voltage boundary within which a power switching device can be operated without destructive failure, and is typically specified for both short-circuit (SCSOA) and reverse-biased (RBSOA) operating conditions.
One such high power switch is an insulated-gate bipolar transistor (IGBT) which employs a trench-gate structure; such a device is described in detail in H.-R. Chang and B. Baliga, xe2x80x9c500-V n-Channel Insulated-Gate Bipolar Transistor with a Trench Gate Structurexe2x80x9d, IEEE Transactions on Electron Devices, Vol. 36, No. 9, September 1989, pp. 1824-1828. The use of a trench gate structure enables the IGBT to have a very high xe2x80x9cinversion channel densityxe2x80x9d, which is defined as the number of inversion channels per unit area. This results in the device having a very high saturation current level. Unfortunately, when such a device is short-circuited, the high saturation current level can result in the device""s destruction; as such, the device""s SCSOA tends to be poor. One possible solution to this problem is the use of external short-circuit protection circuitry; however, such circuitry adds undesirable cost and complexity to the device.
Another power semiconductor switch is described in U.S. Pat. No. 5,329,142 to Kitagawa et al. This device employs a number of IGBT structures, with each IGBT separated from its adjacent IGBT with several trench gates and xe2x80x9cfloating mesasxe2x80x9d (i.e., areas between adjacent gates capped with an oxide layer). The IGBT mesas and the floating mesas are equal in width, and all are narrow. The use of floating mesas and a deep trench tends to improve the device""s VFD; however, the narrow mesas results in a high channel density, and thus a very high saturation current level and a poor SCSOA.
Furthermore, the deep trenches cause the switch""s reverse breakdown voltage characteristic to be poor. When the device is reversed-biased, the deep trenches cause the reverse-bias-induced electric field to be non-uniform with very high peaks at the corners of the trench bottoms, which can lead to the premature breakdown of the device.
The described device also employs P base regions which terminate well above the bottoms of the trench gates. This also contributes to the formation of a non-uniform reverse-bias-induced electric field with high peaks, and a poor breakdown characteristic.
An insulated-gate bipolar switch is presented which overcomes the problems noted above. The new switch is particularly well-suited to high-power switching applications, providing simple gate voltage control of switching, low VFD, and robust SCSOA and RBSOA, using a simplified device fabrication process.
The novel structure contains a number of trench-IGBT structures, but reduces the inversion channel density of a conventional trench-IGBT device by interdigitating a number of floating mesas and trench gates between IGBT structures. The reduced inversion channel density lowers the saturation current level (when compared with an all-IGBT implementation) and thus improves SCSOA. The IGBT mesa widths and the floating mesa widths are made non-equal, which enables the switch to provide desired VFD, SCSOA and RBSOA characteristics.
In a preferred embodiment, the IGBT mesas are wide and the floating mesas are narrow. The accumulation channels which form along the bottoms of the trench gates enhance electron injection efficiency and thereby keep the switch""s forward voltage drop VFDlow. The narrow floating mesas improve the uniformity and lower the peaks of a reverse-bias-induced electric field, thereby improving RBSOA, which is further improved by making the depths of the trench gates equal to that of the P base regions in the IGBT mesas. The mesa widths and the number of floating mesas per unit cell are adjusted as needed to provide a device having a desired set of characteristics.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.